From measurement of either the intracellular transverse (2) or longitudinal (6) relaxation times under circumstances in which the extraceilular relaxation rates are made variable, the mean residence time (Ta) for water molecules within the cell can be determined. The effect of addition of Mn2+ ions is to decrease the relaxation times of extracellular water to values much shorter than those of intracellular water. When a water molecule moves through the membrane into extracellular space its spin state can be "relaxed" within a few msec after it has diffused an average displacement of a few ,tm. It is in this respect that the problem of extracellular unstirred layers is removed.A second NMR experiment that also hastens relaxation in the extracellular region is the pulsed field gradient spin echo experiment (17). This experiment, normally used to measure the diffusion coefficient of water, can be utilized to determine the value of T, (19,20

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